Ceramic transducer having stacked elements



April 2, 1957 w. CAMP CERAMIC TRANSDUCER HAVING STACKEID ELEMENTS Original Filed March 6, 1951 NON-ELECTRO-MECHANICALLY R SIVE.

NON-ELECTRO-MECHANICALLY RESPONSIVE I N V EN TOR.

Leon W. Camp CERAMIC TRANSDUCER HAVING STACKED ELEMENTS Original application March 6, 1951, Serial No. 214,144.

Divided and this application February 18, 1952, Serial 7 Claims. (Cl. 340-) This invention relates to transducers for converting electrical oscillations into traveling compressional waves in fluids and vice versa, and has to do particularly with transducers for this purpose employing electro-mechanically responsive ceramics such as barium titanate.

These ceramics resemble piezo electric crystals in that they deform physically in response to electrical potential and vice versa, but have the great advantage over piezo electric crystals in that they are not so limited as to shape or size, are very rugged, and are relatively eflicient converters of energy from electrical to mechanical form and vice versa.

This is a division of my application Serial No. 214,144, filed March 6, 1951, and titled Ceramic Transducers for Underwater Sound Transmission and Reception.

An object of the invention is to provide ceramic transducer designs utilizing the special properties of electromechanically responsive ceramics to high advantage in converting traveling waves in fluids to electrical oscillations and vice versa.

Other more specific objects and features of the invention will appear from the following description with reference to the drawing, in which:

Fig. l is a longitudinal sectional view of a transducer employing a stack of ceramic disc elements;

Fig. 2 is a side elevation view showing a variation of the structure of the ceramic element of Fig. 1;

Fig. 3 is a view taken in theplane III--III of Fig. 2; and

Fig. 4 is a view similar to Fig. 2 showing a modification thereof. 7

The present invention is directed not to any particular ceramic composition, but to shapes and methods of mounting ceramic elements to obtain efiicient transducers. Various compositons have been developed and are being developed, but for the purpose of the present invention it is merely necessary that the material be electro-mechanically responsive and be capable of being formed into the shapes disclosed. Such shaping may be done by molding or extruding the material in plastic form prior to firing, or by cutting or machining after firing. The material is polarized after firing by applying a high unidirectional electric field in the direction of desired polarity, which is between the working electrodes, so that the latter can be used to apply the polarizing potential.

Barium titanate is a common basic ingredient of presently known ceramic electro-mechanically responsive elements, and for disclosures of compositions of some electro-mechanically responsive ceramics that may be used, reference is made to Wainer Patents 2,402,515, 2,402,516 and 2,402,518, but the present invention is in no sense limited to those particular compositions. For convenience, electro-mechanically responsive elements capable of being formed into shapes disclosed herein will be referred to hereinafter simply as ceramics.

It is a characteristic of presently known ceramics that the direction of the primary mechanical forces produced 2,787,777 Patented Apr. 2, 1957 by an electrical field therein is parallel to the field, in contrast to piezo-electric crystals in which the primary mechanical force is perpendicular to the field in most sonic and supersonic applications. The present invention, in some of its forms, utilizes this characteristic of ceramics to full advantage.

The transducers herein disclosed are particularly adapted for the transmission or reception of supersonic vibrations in water for use in depth sounding, underwater ranging, etc., but since they are not limited to any particular frequency range, the vibrations will be referred to, for convenience, as sound, which term as used herein includes both the sonic and supersonic range of frequencres.

Referring first to Fig. 1, there is disclosed a transducer consisting of a fluid tight case 20 closed at the front end by a rubber cap 21 which has bonded thereto and supports a vibrator member 22. The rubber cap 21 is preferably of a rubber having substantially the same soundtransmitting properties as the water or other fluid in which the transducer is to be immersed, so that it acts as a sound Window to transmit vibrations or sound from the vibrator element 22 through the window with substantially no reflection. The cap 21 is shown as secured in fluidtight relation to the end of the case 20 by a band clamp 23.

The vibrator 22 consists of a stack of ceramic elements in the form of discs 24 bonded together by thin films of electrically conducting material such as silver. The faces of the discs 24 may be coated with silver paint prior to assembly of the stack, and then placed together in the stack, and pressed together while subjecting them to heat in a furnace to fuse the silver and bond the discs together in a solid member. The silver films on the outer ends of the end discs function as electrodes, whereas those between the discs function not only as electrodes but as bonding elements. The film on the front face of the front ceramic disc 24 may be mechanically bonded to the rubber cap 21 with any suitable adhesive, many of which are well-known in the art. One set of alternate electrodes 25 are connected together and to a circuit exterior of the case 20 by a lead 26, and the other set of alternate electrodes are connected together and to the external circuit by a lead 32. With this connection the different ceramic discs 24 are connected in parallel, electrically. This has the substantial advantage of presenting a low impedance to the circuit connected to the leads 26 and 32, and make feasible a vibrator of this general type. Thus if the vibrator consisted of a single, long cylinder of ceramic with only two electrodes at opposite ends thereof, the impedance of the unit would be extremely high, and it would be difiicult to match the impedance to conventional electrical circuits.

As has been previously indicated, the primary mechanical forces developed in a ceramic are parallel to the electrical field therein; therefore the primary vibratory forces developed in the unit 22 are longitudinal, which is the useful direction. For maximum efiiciency, the overall length of the vibrating member 22 should be onehalf wave length at the desired frequency of operation. Under these conditions, member 22 vibrates longitudinally at its natural or resonant frequency by alternate compression and expansion, and has much greater amplitude of movement than at other frequencies. Such vibration develops severe compression and tensile forces in the body, and the tensile forces can be resisted only by a very strong bonding between the elements. The necessary strength can be provided between ceramic elements by the bonding of silver fused thereto.

The use of an elongated vibrator vibrating longitudinally, and constructed of a stack of relatively thin discs connectedin parallel, as shown, is advantageous not only becauseit gives a high working cfiic'iency with a'low impedance, but also facilitates "therpolarization of the ceramic material because of the low impedance of the mass of ceramicmaterial between each pair of adjacent electrodes 25.

Fig; 2 shows a vibrator member 22a differing from the member 22 in Fig. 1 only in that it haslongitudinal holes 28 extending therethrough, and in having an end" plate 27 consisting of a solid rigid disc of some material such as aluminum. The function of the plate 27 is to present a continuous surface to the rubber window 21 (Fig. l) and transmit the vibrations of the end of the member 22a to the window as a continuous piston.

'The advantages of the vibratory member 22a over member 22 are as follows: The percentage of sound transmitted through the contact boundary of two mediais governed by the relative densities of the two media. The structure of member 22a quite effectively reduces the effective density of the vibrating member, thereby improving the efficiency of transmission. .This also lowers the mechanical Q of the vibrating member and broadens the frequency band over which it operates at maximum efiiciency.

Fig. 4 shows a vibrator member 22b diifering from member 221: of Fig. 2 only in having a rear end plate 30 ofrigid material bonded to the adjacent ceramic disc, to provide an'inert (with respect to e'lectro-mechanical properties) mass at the free end of the stack. The plate 30 is shown in the drawing as a disc ofthe same diameter'as the ceramic discs 24b, but this is not essential. The plate 30, by virtue of its mass, alters the resonant frequency of the vibrator, and increases the amplitude of vibration of the front (active) end. For the latter purpose, it may be made heavier than the plate 2711 at the front end. It also reduces the Q of the vibrator, which is .desirable in some applications.

The use of the inert rear end mass plate 30 is not restricted to the specific vibrator of Fig. 2, having apertured. discs, but is also applicable to the vibrator of Fig. lhaving solidceramic discs, and with or without an inert mass element (such as the plate 27 in Fig. 2) at the front end.

It will be understood that although in the drawing the bonding elements .25, 25a, 25b have been shown of appreciable thickness, actually .theyare extremely thin, too thin .to show in their proper relative dimensions in a drawing of thisscale.

Although for the purpose of explaining the invention,

.sides of an;electric circuit.

a particular embodiment thereof has been shown and described, obvieus'modific'anons will occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

I claim:

1. In a transducer: an elongated longitudinally-vibratile vibrator "member consisting. of allongitudinal' stack of solid elements bonded together for vibration as an integral body by expansion and contraction; means bond ed .to one end ofsaid member for supportingit with its said one end in vibration-translating relation with a fluid medium; the'other end of said member being free for unrestrained longitudinal vibrationat the natural frequency of "said member; one at least of said elements consisting of electrornechanically-responsive ceramic dielectric 'material having conductive;electrodes.onits end faces; and means for connecting said electrodes to the two respective -2. A transduceraccording to claim 1 in which a plurality of said elements consist of electromechanically-- responsiveceramicdielectric material and said conductive electrodes on juxtaposed faces 'of two adjacent ceramic elements consisting ofa metallic. film fused to said juxtaposed faces and constituting a mechanical bond therebetween.

3. A transducer according to claim 1.in which said dielectric element has a longitudinal passage extending therethrough.

4. A transducer'according to claim 1 in which said member comprisesarigidirnperforateelement at said one end.

5. A transducer accordingto claim 1 in which said member comprises an electromechanically-inert mass 616? ment at one end. w

6. Apparatus according-to claim 1 in which said member comprises an electromechanically-inert mass element at each end thereof. p

-7. A transducer according to claim 1 in which said dielectric element has. aplurality of longitudinal passages extending therethrough.

References Cited in the file of this patent I UNITED STATESPATENTS 1,450,246 Cady Apr. 3, .1923. 2,233,992 Wycofl' Mar. 4, 1941 f 2,271,200 Mason. Jan. 27, 1942 2,426,650 Sivian Sept. 2, 1947 V Klein Sept. 5, 19 

